The present invention relates to a method of coating a coating material, a method of manufacturing an element, a base material to be coated, a method of manufacturing a base material to be coated and an apparatus of coating a coating material including a base material to be coated, and in particular, to those capable of coating a resist on a base material having a curved surface and obtaining uniform distribution of coating thickness.
There has been known the so-called spin coating for spin-coating a coating material such as a resist on a plane surface on a base material such as Si base plate, in, for example, light lithograph or EB (electronic beam) lithograph.
In this spin coating, a droplet of a resist solution is dropped on the neighborhood of the center of the base material in a flat board shape and the base material is rotated, thus, the resist is spread to coat on the surface of the base material by the centrifugal force caused by the rotation of the base material, and excessive resist is shaken off. Incidentally, the distribution of coating thickness of the resist on the base material is determined based on physical properties (viscosity, surface tension and others) of the resist and on a speed of rotation of a rotary member (spin coater) in the case of rotating the base material as well as ambient conditions (temperature and others).
In the spin coating described above, when one surface of the base material to be coated is a plane, it is possible to obtain the distribution of coating thickness which is mostly uniform. However, in the case of the same spin coating on the base material having a curved surface on one surface thereof, it has been impossible to obtain the uniform distribution of coating thickness. Namely, in the case of resist coating on base material 200 having a shape of a curved surface shown in FIG. 22(A), there have been caused areas having uneven coating thickness.
In addition, when conducting spin coating, a droplet of a resist solution is dropped on the surface of a base material, and then, the base material is rotated at a prescribed speed of rotation for preliminary spinning after the surface is covered, and after that, the regular spinning is conducted at a prescribed speed of rotation.
However, when spin coating has been conducted by the aforementioned method on a base material having a shape of the curved surface, there has been caused a problem that a coating thickness is small on the top portion of the base material, a tendency of monotone increase of coating thickness distribution becomes more conspicuous as the location moves from the rotational center of the base material to the peripheral portion, and the coating thickness cannot be make uniform.
In particular, a specific shape represented by a curved surface shape has made it impossible to remove the change in coating thickness resulting from an influence of gravity applied on the resist.
Further, for obtaining a desired coating thickness, it is conceivable that a resist coating process and a baking (heating) process are respectively repeated plural times. In this case, however, a difference of portions of uneven coating thickness was enlarged each time the process was repeated, because a portion of uneven coating thickness caused by nth resist coating was superposed on the area of uneven coating thickness caused by the first resist coating.
It is also conceivable that a flat portion is formed along the circumference of a curved surface portion of the base material to improve the flow of a droplet caused by centrifugal force, so that formation of uniform coating thickness is urged in spin coating. However, this method has caused a problem that a size of the base material (the basic material) in the manufacturing stage is large, resulting in increase of members to be used and increase of cost, and machining of a base material takes a long time, resulting in a long term of works.
Further, when obtaining a shape of the base material 200 through cutting by a general purpose lathe, plural lines TM in a shape of concentric circles which are called tool marks are undesirably formed, as shown in FIG. 22(C). That is, the base material chucked by a chuck on the lathe is rotated, and a tip of a cutting tool is brought into pressure contact with the rotating base material to cut the base material by moving continuously from the peripheral portion, thus, an area that is touched by the cutting tool is generated on the peripheral portion, and tool marks in the shape of concentric circles are formed on the surface of the base material.
Due to the tool marks thus formed, roughness caused by cutting by the cutting tool is reflected as it is as “surface roughness of the base material”, and the surface roughness on the KD portion of base material 200 in FIG. 22, for example, becomes a surface form of the base material at about 600 (nm), for example, therefore, it is difficult to control displacement of coating thickness distribution obtained by coating resist finally to be less than allowable roughness. “Surface roughness” in this case means the one indicating value Ra obtained by the expression shown in the drawing, when a certain surface is defined as roughness curve f(y), then, a portion corresponding to measured length 1 in the direction to the center line is sampled from the roughness curve f(y), and when the roughness curve is expressed by x=f(y) with X axis representing the center line of the sampled portion and with Y axis in the direction of longitudinal magnification as shown in FIG. 22(B). Therefore, if the value Ra representing the surface roughness takes the value stated above, distribution of coating thickness of resist coating is also affected.
In particular, because of the tool marks which are formed, when measuring a coating thickness optically, light is subjected to diffused reflection by grooves of the tool marks, which has made it impossible to measure coating thickness and has caused troubles for measurement and evaluation of coating thickness.